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Old 29th October 2011, 11:17 PM   #2961
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Maybe i am nuts but that could actually work too.
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File Type: pdf Starless Transconductance.TSC - TINA.pdf (14.6 KB, 117 views)
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Old 30th October 2011, 10:34 AM   #2962
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Is R15 1M correct and is the output impedance as low as say 240k at this point?
If you want to use 10n for C7 I arrive at R15 218k7, R16 31k8 and C8 3n43.
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Old 30th October 2011, 01:05 PM   #2963
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See the simulation in post 2929. I put in the same values. It has to be seen if the helper transistors in post 2961 raise the output impedance.
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Old 30th October 2011, 03:09 PM   #2964
MiiB is offline MiiB  Denmark
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This last circuit does away with the risk of frying the cart....I like that...as it also removes any adjustments from the input.....
I like your servo that sense at the output and injects before the buffer better
I see the change of gain with different carts, but I don't see it as a problem as is is possible to change gain with by jumpering different RIAA networks.
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Old 30th October 2011, 05:17 PM   #2965
sampler is offline sampler  Lithuania
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Well, today I have listened to op amp version. Servo worked as expected so it was dc coupled. At first glance everything was ok, plenty of gain, but after couple records I realized that something was gone, something that was so appealing about this circuit - ultimate transparency and micro details. Clearly OP275 is not up to the task here, even though it is really not a bad op amp, jfet (or bipolar hybrid, I should check...) input and so on. So I reverted back, mmmm.... much better
I sim'ed current mirrors with helper transistors and if multisim is not lying, output impedance raised from 320k to 717k, so this gives me nice RIAA caps values and good bump in gain. This could be enough, I'm going to check it now.
About your last transconductance circuit. At first look I have some doubts. I think input pair would not conduct, it would just float, and grounding sources would leave only one current mirror working, but maybe I'm wrong, I will check it later.
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Old 30th October 2011, 07:02 PM   #2966
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Maybe post 2961 works when you float the cartridge between gate and source of the input Fet, ground not connected. At the source there should still be nearly zero DC.
Anyway, nice that you can get more gain with the helper transistors. In the Opamp version you could try to flip the leads at the cartridge. Maybe the phase inversion results in the strange sound you get. Anyway, if you get it going without the Opamp thats more elegant.
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Old 30th October 2011, 07:33 PM   #2967
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Like this :
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File Type: pdf Starless Transconductance Floating.TSC - TINA.pdf (14.6 KB, 71 views)
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Old 30th October 2011, 09:32 PM   #2968
RCruz is offline RCruz  Switzerland
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I keep getting your pdf´s in three pages. The middle page has the schematic and the others have some components designations (all in a bundle... unreadable) is this avoidable ?
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Old 30th October 2011, 09:47 PM   #2969
RCruz is offline RCruz  Switzerland
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Quote:
Originally Posted by sampler View Post
It's dead quiet Joachim, only broadband noise (2mV/div in foto). Yes, you are right, in my case resulting gain is ~52dB. With 6Ohm cart. that would be way much more. I attached circuit as build and some sims, any tricks and tips would be appreciated. I really like it, I'm even considering upgrading my preamp for more gain if that's the last resort.
I see you started new thread for nobrainer. That looks elegant/interesting too, I may give it a try, just were did I put those low noise 2sc/2sa...
Hi Sampler

What is the 100k trimmer (R3) needed for in your Starless version ?
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Old 30th October 2011, 09:53 PM   #2970
RCruz is offline RCruz  Switzerland
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Quote:
Originally Posted by Joachim Gerhard View Post
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Hi Joachim

Looking at your last schematic, I estimate the first stage output impedance should be around 286k... am I correct ?

The opamp arrangement on the output acts just as a servo or does it have amplification ?
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